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Is this typical idle behavior?

Omega81

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Sep 13, 2019
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Hello. New to the forum. I have a 2004 Jeep Wrangler 4.0 5 speed manual I recently bought. Before I starting having problems with the engine. If I went up a hill at idle the engine would compensate under the load and increase idle/power. So my question is is this typical? If so what controls that idle under load? My old engine started knocking pretty bad, tore apart couldn't salvage it. Ended up putting in another 4.0 with 30k. But this new 4.0 does not idle under load like my old engine did. Any thoughts would be appreciated. Thank you.
 

Camhabib

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Mar 30, 2019
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What do you mean “idle under load?” Idle implies there’s no load on the engine, otherwise that isn’t an idle. It’s like saying “resting heart rate during exercise.”
 
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Omega81

Omega81

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Example. I'm at the base of a incline. 1st gear, at idle. Slowly let off the clutch, no gas. Start moving forward with idle. Start to climb up the incline with idle. Engine starts to bog. Then increases idle/power to compensate for the load.
 

Camhabib

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Example. I'm at the base of a incline. 1st gear, at idle. Slowly let off the clutch, no gas. Start moving forward with idle. Start to climb up the incline with idle. Engine starts to bog. Then increases idle/power to compensate for the load.
I’m still a little confused. The only way your RPM should increase on an engine without applying gas is by increasing wheel speed (which is how I used to start my first motorcycle, pushing down a hill and popping in the clutch). I’ve also never driven a manual TJ before, but I’ve never seen a vehicle that wouldn’t stall if you tried to release the clutch without giving it gas going uphill, no matter how slowly you let the clutch out. It sounds like your old TJ just idled far too high?
 
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Dougj169

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Jun 10, 2019
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Idalia Colorado
As far as I know all computer controlled engines the idle is set at the ecm and it will compensate for the rpm drop under load. If it is not a drastic change it will keep chugging along
 
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Gollywomper

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Feb 9, 2019
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Corning CA
Search on the idle air control valve. Could need a cleaning they get gummed up and dirty. Are you using the same throttle body as on the old engine?
 

Gollywomper

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Corning CA
On mine, idle in neutral is about 750rpm. Rolling on flat ground while idling is somewhere just above 1000rpm.
Mine does the same thing, as soon as I come to a stop it returns to 750. Any idea why they do this? I know of others that do not act like that.
 

jjvw

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Feb 17, 2018
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Mine does the same thing, as soon as I come to a stop it returns to 750. Any idea why they do this? I know of others that do not act like that.
No idea. I'm assuming mine is doing what it is supposed to do.

Very rarely, mine will idle in the 600s and I can tell that is too low by how rough the engine gets.
 
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Omega81

Omega81

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Search on the idle air control valve. Could need a cleaning they get gummed up and dirty. Are you using the same throttle body as on the old engine?
No. I'm using the throttle body that came with the new engine. Thank you. I'll look into the IAC.
 
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Omega81

Omega81

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As far as I know all computer controlled engines the idle is set at the ecm and it will compensate for the rpm drop under load. If it is not a drastic change it will keep chugging along
Ya. That's what my old engine did. It just refused to stall and apply idle accordingly.
 

Goatman

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If you have the A/C on at a red light you have idle under load. That is why when you turn the A/C on the engine picks up a few rpm on it's own. An engine running a bit rich can accommodate an added load easier than when running lean. I think this is why some folks will stall while others can let the engine pull them without stepping on the gas pedal.
 
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